Solr index URL should be configured under the main "index" section

Configuration of the Solr index URL must currently be done in a
subsection, as follows:

  [index]
    type = SOLR
  [index "solr"]
    url = 127.0.0.1

Subsections are also being used for configuration of specific indexes
like the "changes_open" and "changes_closed" subsections that are used
for Lucene indexes, and are documented as "index.name.xyz".

Having the Solr URL configration under a similar subsection may cause
confusion, and it would be more intuitive to configure the URL under
the main index section, like:

  [index]
    type = SOLR
    url = 127.0.0.1

Subsections are being used for configuration of specific indexes like
the "changes_open" and "changes_closed" settings that can be specified
for Lucene indexes which are documented as "index.name.xyz"

Also add missing documentation of the URL parameter.

Change-Id: I0bd9eabbdbfa29241688c738aa5d213e726c90b0

Documentation/config-gerrit.txt

@@ -1901,6 +1901,12 @@ Gerrit server.
 Open and closed changes are indexed in separate indexes named
 'changes_open' and 'changes_closed' respectively.
 
+[[index.url]]index.url::
++
+Only used when the type is `SOLR`.
++
+URL of the index server.
+
 [[index.name.ramBufferSize]]index.name.ramBufferSize::
 +
 Only used when the type is `LUCENE`.

gerrit-solr/src/main/java/com/google/gerrit/solr/SolrChangeIndex.java

@@ -100,9 +100,9 @@ class SolrChangeIndex implements ChangeIndex, LifecycleListener {
     this.indexes = indexes;
     this.schema = schema;
 
-    String url = cfg.getString("index", "solr", "url");
+    String url = cfg.getString("index", null, "url");
     if (Strings.isNullOrEmpty(url)) {
-      throw new IllegalStateException("index.solr.url must be supplied");
+      throw new IllegalStateException("index.url must be supplied");
     }
 
     // Version is only used to determine the list of stop words used by the